The present invention relates to a method and an apparatus for performing a visual field test. It also relates to computer-readable media which store a computer program for processing a result of one or more visual field tests.
Visual field testing is one of the most important examination methods in eye care. It is used for examining the range and the sensitivity of a subject""s visual field in connection with diagnosis and treatment of different diseases of the eye, e.g. glaucoma and retinal disease, and of the central nervous system, e.g. brain tumours and inflammations of the optic nerve.
Today most visual field tests are performed with the aid of special computerised instruments (the so-called perimeters) measuring the limit or threshold of a subject""s perception of light at a number of test locations in the subject""s visual field.
Typically, the test is performed in the following way. The patient is seated in front of a perimeter and asked to look steadily at a centrally placed fixation target, e.g. on a screen or in a hemispherical bowl. Visual stimuli are presented successively with different intensities and at different locations. The patient is asked to press a response button every time he perceives a stimulus, whether close to or distant from the fixation target, whether faint or strong. Alternatively or additionally, the perception of stimuli can be recorded by objective methods, such as measurement of electric potentials in the brain or in the eye of the patient (VER perimetry and ERG perimetry, respectively) or recording of pupil reactions (pupil perimetry).
There are different strategies for selecting test locations and intensities of the stimuli presented at these test locations in order to establish a patient""s threshold for perception of light. In one common method, the so-called staircase method, a stimulus which has an intensity close to the expected threshold value at the test location concerned is shown. If the patient does not respond to the stimulus, the intensity of the subsequently presented stimuli is thereafter increased stepwise until a response is received from the patient, i.e. until a stimulus is seen. The first intensity level at which a response is received may be defined as an estimate of the threshold of the test location concerned. The precision of the test can be increased by reversing the test process when the first response is received, and by continuing it in smaller steps with decreasing intensities until the first unseen stimulus is encountered. The threshold may then again be estimated as the intensity level of the last seen stimulus. If, on the other hand, the patient responds to the first stimulus, the intensity is decreased stepwise until no response is received, whereupon the test procedure is reversed.
There are several known strategies of how to vary the intensities of the visual stimuli depending on the response to previous stimuli in order to obtain a rapid and accurate determination of the threshold value of each test location.
In order to obtain more accurate threshold estimates, responses to the visual stimuli at all test locations can also be processed by statistical methods. See e.g. U.S. Pat. No. 5,461,435, wherein the maximum of a posterior probability function is used for calculating the best estimates of the threshold values during and after the test. See also the article xe2x80x9cScandinavian Journal of Statistics, Vol. 21, No 4, p 375-387, 1994, Olsson J and Rootzxc3xa9n Hxe2x80x9d, which describes another particular method of estimating the best threshold value of each test location from the estimated distribution of threshold values of each test location, namely MPM or Marginal. Posterior Mean which is achieved by calculating a mean threshold value from the posterior probability distribution of threshold values.
The geographical distribution of points showing reduced sensitivity is of great clinical importance, and it is, therefore, very important that this geographical information is displayed to the user in an easily comprehensible way. A common way of displaying the result is to show the numerical value of the single best threshold estimate at each test location in a map. An example of such a map is shown in FIG. 1. Such numerical maps are difficult to use. Greyscale representation facilitate the usage to some extent.
The result of the visual field test may also be shown as a deviation map that shows the deviation of each measured threshold value from the corresponding threshold value of a mean age-corrected reference field or a significance map that shows the significances of these deviations. FIGS. 2 and 3 show an example of a deviation map and a significance map, respectively. The deviation maps and the significance maps are advantageous in that they show deviations from expected normal values and the significances of such deviations.
The result of a visual field test can also be presented in the form of a so called visual field index, which may e.g. be obtained by averaging the calculated threshold values of all test location.
All test methods developed so far have been focused on obtaining one or two good estimates of the threshold value at each test location using as few visual stimuli as possible. The threshold values which represent these estimates are displayed, either directly or in refined versions, and used for assessing whether the measured visual field deviates from a normal field or not.
However, it has been established that threshold values from the same test location of the same eye, exhibit variability within a test and between tests often called short term fluctuation and long term fluctuation, respectively. In addition the estimation of the threshold value is always associated with a measurement error. This measurement error is influenced by many factors, e.g. the frequencies of false positive and false negative responses, loss of fixation and variations in stimulus response times. Therefore, a single threshold value estimate determined at a certain test location at a certain moment during the test does not give a complete representation of the patient""s capability of perceiving light at that point of the visual field. Thus, there is a risk of erroneous conclusions with the present methods.
One object of the present invention is to suggest a method and an apparatus for performing a visual field test, which method and apparatus give a better representation of the visual field of the test subject.
Another object of the invention is to suggest such a method and apparatus, which process the responses received from the test subject during a visual field test such that the user obtains more information of the measured visual field, thereby facilitating the interpretation.
At least one of these objects are achieved by a method, an apparatus and computer-readable media having the features of claims 1, 15, 26, and 31, respectively.
The present invention is based on the understanding that the estimation of the threshold values is more or less certain due to estimation errors as well as short and long-term fluctuations, that the degree of confidence varies between different test locations, with normality and with disease, and that a better representation of the measured visual field would be obtained if the confidence of the test result was estimated, used in the subsequent testing and/or conveyed to the user.
In a first aspect, the present invention relates to a method of performing a visual field test, comprising the steps of successively presenting a plurality of visual stimuli to a test subject, each visual stimulus being presented at a test location; recording, for each visual stimulus, any response thereto from the subject; and calculating, on the basis of the recorded responses and with the aid of a confidence level function, at least two visual field test values of different confidence levels of the test subject""s perception of visual stimuli, said at least two visual field test values being calculated on the basis of the same recorded responses.
The presentation of visual stimuli and the recording of responses thereto can be performed as in the prior art. However, the responses are processed in a new way.
More particularly, a confidence level function, c(v), is used to calculate confidence levels of visual field test values, v, e.g. threshold value estimates, t. The confidence level function can be any function which is suitable for establishing values of different confidence levels of the test subject""s perception of visual stimuli. Below examples of different confidence level functions are given.
A threshold value estimate t1 with a certain confidence level Kli is a solution or approximate solution to the equation ci(ti)=Kli of test location i. The confidence level function may have the property of being increasing with threshold value (here denoted xe2x80x9cincreasing confidence level functionxe2x80x9d) i.e.: if t1 less than t2, then it follows that ci(t1)xe2x89xa6ci(t2) (equation (a)).
The values of the confidence level function ci(t) may depend on all data recorded from the patient such as responses, fixation etc and all the data recorded on the perimeter such as stimulus levels, time of stimulus and so on. These data are denoted test data.
The confidence level function ci(t) could e.g. indicate the probability that the threshold is located in the interval (xe2x88x92,t] i.e. equal to or smaller than t (such a probability satisfies equation (a)) or in the interval (t,). Such a function is here denoted xe2x80x9cprobability type confidence level functionxe2x80x9d.
An example of the threshold estimate calculations in the probability type confidence level functions is the following. Let ci(t1)=Kli=50%, and assume that ci(t) is a xe2x80x9cprobability type confidence level functionxe2x80x9d indicating the probability that the threshold is in the interval (xe2x88x92,t]. In this case t1 is a threshold estimate such that there is a 50% probability in the Bayesan sense that the actual threshold is below t1.
A different and alternative confidence level function is the following. Compute the probability that a certain function of the test data gi=gi (data) attains a value in the critical region Ci (which may depend on the test data). If it is assumed that the patient has a patient behaviour or part of a patient behaviour which is characterised by the parameter x, this probability is denoted with P(gixcex5Ci|x). In general, the true value of the patient parameter is unknown and P(gixcex5Ci|x) is calculated for hypothetical values of x. Functions which depend on P(gixcex5Ci|x) are here denoted xe2x80x9ctest type confidence level functionxe2x80x9d. For instance with Ci=(t,b] or (a,t], intervals on the real line, and x denoting a certain patient behaviour ci(t)=1xe2x88x92P(t less than gixe2x89xa6b|x) and ci(t)=P(a less than gixe2x89xa6t|x) are test type confidence level functions that satisfy equation a.
It is possible to calculate other confidence level function from l(t)/L, wherein l(t) denotes the likelihood function, i.e. the probability of obtaining the observed test data given that the threshold is t, and L is the supremum of l(t) over all values of t. This is here called a xe2x80x9clikelihood type confidence level functionxe2x80x9d. In this case the confidence level may be non-increasing.
If the confidence level function is not strictly increasing then ci(t)=Ki may have several solutions and there are more than one threshold estimate with the same confidence level. In this case, a rule for combining the different threshold estimates is needed.
More than one confidence level function may be used at one and the same test location. It is conceivable that confidence levels K1i, K2i, . . . are calculated using test data.
Confidence level functions may also be defined for other arguments than a single threshold, t. For instance confidence level functions, ci(di), expressing the confidence in a deviation, di=tixe2x88x92ni, of a single threshold, ti, from the corresponding threshold value of a mean age-corrected reference field, ni. In the single test a number, N, of confidence level functions ci(f(t1,t2, . . . ,tn)), for i=1, . . . , N, may be calculated for functions, f(t1,t2, . . . tn), of thresholds from n test points (such functions are often called visual field indices). An example of a single test confidence level function is f(t1,t2, . . . ,tn)=xcexa3i=1n(ti/n), the average threshold value. When analysing results from more than one visual field test, confidence level functions ci(xcex94i) may be defined for e.g. differences, xcex94i=ti,2xe2x88x92ti,1, between threshold values ti,1 and ti,2 of the same test point measured at a first and a second visual field tests. Another example of analyses using data from several visual field tests is, with xcex2=f(t1,1, t2,1, . . . , tn,1, t1,2, t2,2, . . . , tn,2, . . . t1,m, t2,m, . . . , tn,m) (where ti,j is the threshold value of test point i at visual field test j) estimating a rate of change, a confidence level, function ci(xcex2) that may be used to express the confidence in xcex2, the rate of change. Thus the use of confidence level functions also applies to functions of threshold values from one or several visual field tests. It can also be similarly applied to any visual field test value indicating the test subject""s perception of visual stimuli, e.g. the gradient of the frequency-of-seeing curve.
Furthermore, perception as used in this application should be taken to include perception measured by subjective methods, e.g. pressure of a response button by the test subject, as well as perception measured by objective methods, e.g. measurement of electric potentials in the brain or in the eye of the test subject.
The two visual field test values of different confidence levels can be used internally in a test algorithm, for determining how to proceed with the current test or a later test. In a preferred embodiment the method comprises, however, the further step of displaying a result of the visual field test and an indication of the confidence thereof, said result and said indication of the confidence being based on the at least two visual field test values of different confidence levels of the test subject""s perception of visual stimuli.
In a preferred embodiment, the step of calculating comprises calculating at least two visual field test values of different confidence levels for each test location. Thus at least two sets of visual field test values are obtained for the visual field of the test subject. The at least two visual field test values of a test location could be defined as two alternative results of that test location since they are calculated from the same responses.
The result of the visual field test may be given in the form of e.g. a threshold value for each test location or a visual field index as in the prior art, or in any other suitable form.
The indication of the confidence may be any indication which gives the user an idea of certainty/uncertainty of the result. It may for instance consist of at least one alternative result of the visual field test. Since the alternative results have different confidence levels, the user gets an idea of the confidence of the result from the difference between the alternative results.
The visual field test values may be values indicating the test subject""s perception of visual stimuli in the whole visual field, a part of the visual field or at single test points. The visual field test values may e.g. consist of visual field indices, which are displayed as the result of the visual field test and the indication of the confidence thereof. The visual field index may be e.g. the mean value of a set of visual field test values or a distribution value, e.g. the standard deviation of a set of visual field test values.
As another alternative, the number of acceptable/unacceptable visual field test values in a set of values of a first confidence level may be determined and displayed as the result of the visual field test, whereas the number of acceptable/unacceptable visual field test values in another set of visual field test values of a second confidence level may be determined and displayed as the alternative result. Whether the visual field test value of a certain test location is acceptable or not could be determined by comparing it with a reference value.
As yet another alternative, the numerical values of the two sets of visual field test values may be displayed as the result of the visual field test and the indication of the confidence thereof. The two visual field test values at each test location, may, for instance, represent a standard estimation and a pessimistic estimation of the visual field test value. In this case the user may be reasonably sure that the test location concerned is normal if both the standard estimation and the pessimistic estimation indicate that the test location is normal.
In the currently preferred embodiment, three visual field test values which represent an optimistic, a standard and a pessimistic test result are determined.
As another example, the indication of the confidence may consist of an indication that the confidence is sufficient/insufficient. This indication can be determined e.g. by comparing the difference between the two visual field test values of each test location with a predetermined value.
The result of the visual field test may be shown in one single map, wherein the result and the indication of the confidence thereof are shown at each test location, or in several maps, where each map show e.g. the result of a predetermined confidence level. Furthermore, interpolated results may also be shown at any point between the test locations.
The visual field test values may be shown directly, either as explicit numerical values or graphically, e.g. by means of a colour or grey scale, on the map as the alternative results of the test location concerned. They may also be refined and shown, e.g. as the deviations from expected visual field test values, which are based on a statistical normal values of a population, or from other reference values. As a further alternative, the significances of the above-mentioned deviations may be shown in the maps.
A preferred way of calculating the at least two visual field test values comprises the sub-steps of calculating a plurality of visual field test values, sorting said plurality of visual field test values by size, and selecting visual field test values of predetermined ordinal numbers to be used as said at least two visual field test values. This is an advantageous way of calculating the visual field test values since it requires a limited amount of computing capacity, with negligible loss of information.
In a preferred embodiment, the visual field test value is a threshold value of the test subject""s perception of visual stimuli.
The present invention is advantageous in that it gives the user and/or a computer algorithm an indication of the confidence of the test result. This is most valuable since the uncertainty of the test result may vary to a considerable extent between different patients and different tests. Knowledge of the confidence level makes it easier to determine whether the visual field is defective or not. The invention is particularly advantageous when an indication of the confidence of each different test location is determined and displayed, since the confidence of the result may present a large variability between different test locations.
In a second aspect, the invention relates to an apparatus for performing visual field test, which comprises means for successively presenting a plurality of visual stimuli to a test subject, each stimulus being presented at a test location; means for recording, for each visual stimulus, any response thereto from the subject; and, means for calculating, on the basis of the recorded responses and with the aid of a confidence level function at least two visual field test values of different confidence levels of the test subject""s perception of visual stimuli, said at least two visual field test values being calculated on the basis of the same recorded responses.
The responses need not necessarily be evaluated in immediate connection with the testing of the test subject. Instead, the responses to the visual stimuli may be stored in a memory and evaluated at a later time and even in a different apparatus or computer. In a third aspect, the invention thus relates to a computer-readable medium having stored thereon a computer program for processing a result of a visual field test, according to which a plurality of visual stimuli are successively presented to a test subject, each visual stimulus being presented at a test location, and any response from the subject to a visual stimulus is recorded, said computer program comprising instructions for a general purpose computer to perform the steps of calculating on the basis of the recorded responses and with the aid of a confidence level function, at least two visual field test values of different confidence levels of the test subject""s perception of visual stimuli, said at least two visual field test values being calculated on the basis of the same recorded responses.
An alternative way of evaluating the visual field test by using the knowledge of the confidence levels of different visual field test values, is to calculate the confidence levels at different test locations of a certain visual field test value. In a fourth aspect, the invention thus relates to a computer readable-medium having stored thereon a computer program for processing a result of a visual field test, according to which a plurality of visual stimuli are successively presented to a test subject, each visual stimulus being presented at a test location, and any response from the subject to a visual stimulus is recorded, said computer program comprising instructions for a general purpose computer to perform the steps of calculating on the basis of the recorded responses and with the aid of a confidence level function, the confidence level of a predetermined visual field test value at each test location; and producing a map of the plurality of test locations, wherein the confidence levels of the predetermined visual field test value is displayed for each test location.
The idea of calculating results of different confidence levels could also be used in the follow-up of visual field tests. In a fifth aspect, the invention thus relates to a computer-readable medium having stored thereon a computer program for processing a result of at least two visual field tests, according to each of which a plurality of visual stimuli are successively presented to a test subject, each visual stimulus being presented at a test location, and any response from the subject to a visual stimulus is recorded, said computer program comprising instructions for a general purpose computer to perform the steps of calculating on the basis of the recorded-responses from said at least two visual field tests and with the aid of a confidence level function, at least two results of different confidence levels for said at least two visual field tests.
By following the development of the visual field on different confidence levels, more and better information could be gained.
The apparatus and the computer programs have the same advantages as described above for the method of the present invention. Furthermore, the features discussed in connection, with the method could also be implemented in the apparatus and in the computer programs. Preferably, all method steps are performed under the control of a computer.